It is known that permanent reconstruction of a bone joint which has been malformed from birth, or as a result of disease, or accidental injury, may be achieved by the substitution of the afflicted portion(s) of the joint by an artificial implant constructed of a biocompatible material. It is desirable that any replacement joint should imitate the natural articulation of the healthy joint.
One method of replacing the shoulder joint has utilised a stemmed humeral prosthesis adapted to be inserted and retained within the medullary canal of the humerus after removal of the natural humeral head. The socket function of the replacement joint is performed by a glenoid cup implant which defines a spherical recess adapted to receive the ball end of the humeral prosthesis. Dislocation of the humeral ball is prevented by the soft tissue structures and the natural stability of the spherical recess. Examples of such implants are disclosed in EP-0216489 and U.S. Pat. No. 4,179,758. However, the dimensions of the glenoid cup are such that there is often insufficient bone tissue to permit the secure fixation of, or the reaming out of, a socket deep enough to contain a more constrained humeral ball.
This problem has led to the design of an `inverted` ball-and-socket arrangement in which the socket is located in the proximal region of the humerus, whilst the corresponding ball articulation protrudes from the glenoid process. This reduces the amount of bone which needs to be removed from the glenoid in order to achieve a more constrained joint. The humeral component consists of a tapering shaft which fits into a reamed cavity within the humerus and on which is located the corresponding socket. A wider range of movements can be achieved using such an arrangement, with additional flexibility being imparted by the mobile anatomy of the scapula of which the glenoid is a part. Examples of such implants are disclosed in EP-0177503, EP-0299889, U.S. Pat. No. 3,916,451, U.S. Pat. No. 3,978,528 and GB 1438950. However, such implants still exhibit a significant reduction in the comfortable range of movements when compared to a normal shoulder joint, resulting in the transmission of considerable loosening forces to the implant/bone interfaces.
Any constrained joint of this type must have a `captured` ball attached to the glenoid via a neck whose minimum thickness is governed by the mechanical strength of the material from which it is manufactured. This neck will interfere with the humerally located rim of the socket and this results in a reduction in the range of movements of the humerus, most notably in full abduction and/or rotation of the humerus. The thinner the neck, the less would be the interference and the greater the available range of movements. Known implants reduce this interference by angling the socket in the humerus, although this is found to compromise the medial movement in order to gain such additional extension. The socket must also surround more than a hemisphere of the ball in order to be constrained, thereby limiting articulation still further.
Furthermore, most known constrained shoulder implants require both the humeral and glenoidal components to be fixed rigidly into their corresponding bones, either by a tight press-fit, cemented or screw attachment. Without the ability to rotate the humerus completely around its longitudinal axis, due to muscle configuration and surrounding tissue, the arm cannot feasibly, or comfortably, be laterally raised from the horizontal to the vertical in a plane parallel to the frontal plane. The arm may be raised to the vertical in the sagittal plane only, and even then this might be expected to result in some abduction of the whole arm, i.e., lateral movement. Such large range of movements mean that impingement of known types of constrained shoulder implants is easily possible. Articular `no-go` areas are potential regions of aggressive dislocation, by leverage, or implant damage if the humerus were forced violently into such an orientation.
U.S. Pat. No. 4,919,669 discloses a shoulder prosthesis of the type comprising a humeral piece intended to be fixed in a humerus to replace at least the head thereof. The humeral piece comprises a hollow tubular element which must be fixed in the humerus substantially along the longitudinal axis thereof and a cylindrical rod, the upper end of which carries a ball cooperating with a glenoid bearing surface to form the shoulder joint and the lower end of which is disposed in said tubular element. The rod is able to slide relative to the hollow tubular element. The glenoid bearing surface consists of a glenoid piece which comprises an anchoring element intended to be inserted into a cavity formed in the scapula and a female hemispherical element. The female hemispherical element cooperates with the male ball of the humeral piece.
British Patent No. 2,223,172 discloses a joint structure for use as a shoulder prosthesis comprising a first member which is implanted in an end of the humerus, a spherical member which is implanted in a socket in the scapula, and an intermediate member which can pivot relative to the first and second members about respective axes. These axes are mutually inclined (preferably at an angle in the region of 45.degree.) and preferably intersect at a position in the intermediate member.